Vascular smooth muscle relaxation is typically hypothesized to be the reversal of activation, i.e. reduction of myoplasmic [Ca2+] myosin regulatory light chain (MRLC) dephosphorylation. We term this form of relaxation as "deactivation" because it is the reversal of activation mechanisms. Some stimuli induce a relaxation that dose not involve "deactivation" mechanisms. Elevations in [cGMP] or [cAMP] can reduce smooth muscle tone despite persistent elevations in MRLC phosphorylation levels. We term this process 'force suppression' to separate it from deactivation mechanisms which reduce force by reducing MRCL phosphorylation.In this proposal, we present evidence suggesting that the mechanism responsible for 'force suppression' involves binding of phosphorylate p20 (also known as heat shock protein 20 or HSP20) to smooth muscle thin filaments. We have preliminary data that recombinant phosphorylated p20 suppresses force in skinned carotid artery. We also have preliminary data that force suppression is not associated with reductions in shortening velocity. This suggests that force suppression is mediated at the crossbridge level. To further study this phenomenon, we propose to study recombinant mutant p20 in skinned carotid artery and to perform detailed muscle mechanics in intact carotid artery. Thee aims are:Aim 1. Test the hypothesis that phosphorylated p20 specifically suppresses skinned smooth muscle force via the actin binding domain contained in the peptide p20 (110-121). This p20 domain has a high sequence homology with a domain in troponin 1 called the inhibitory peptide.Aim 2. Test the hypothesis that force suppression is regulated by p20 phosphorylation at serine 16.Aim 3. Test the hypothesis that force suppression is mediated at the crossbridge level.Inappropriate responses of smooth muscle are involved in most morbidity and mortality in the developed world. Many current treatments target smooth muscle to reduce tone. It is possible that targeting the force suppression pathway (i.e. p20) might provide comparable benefits.